The invention relates to a seat belt pretensioner.
U.S. Pat. No. 5,842,344 teaches a seat belt pretensioner with a piston activated by a pyrotechnic generated propellant gas in a linear direction in a guide arrangement. In this device the propellant gas acts upon the piston in a pressure chamber. To avoid excess pressure in the pressure chamber, the known device has a blowout patch on a front wall of the piston. The blowout patch has points of intended weakness, via which an evacuation passageway is created under excess pressure in the pressure chamber. In a second embodiment, the front wall of the piston has a small calibrated discharge opening to facilitate the desired pressure reduction. In a third embodiment, the front wall of the piston has a tear-off part which, during the return stroke of the piston at the moment of load limitation, is removed from an evacuation passageway.
There is provided by the present invention a seat belt pretensioner activated by opening a gas flow port with a controlled time delay, such that after the tensioning stroke of the piston sufficient time remains for a blocking pawl to engage with a belt reel in a blocking position. A weak point, in the form of a plastic deformable membrane, provides for pressure reduction in the pressure chamber via the gas flow port. Deformation of the membrane to such an extent that a gas flow port is created takes time. During this time a main blocking device, in particular a pawl supported on the retractor frame, can engage the belt reel in a blocking position, in particular with blocking teeth on the belt reel. This ensures a secure blocking of the belt reel. In addition it ensures that a load-limiter can come into effect. As a result, excess forces exerted by the secured seat belt on the vehicle occupant are avoided during the forward displacement of the vehicle occupant.